The present invention relates to an inverter arrangement for supplying an AC output from DC input, and more particularly, to an arrangement including both a first inverter and a second inverter for supplying an AC output with minimal DC content.
Power inverters have long been used to convert DC power into AC power for energizing one or more AC loads. Typically, such inverters include switches such as transistors which are controlled in pulse width modulation (PWM) operating mode to produce a PWM waveform comprising a series of pulses and notches. The waveform is converted into a sinusoidal output waveform by a filter which is coupled to the inverter output. Such an inverter may be used as part of a variable speed constant frequency (VSCF) power generating system in which a generator is driven by a variable-speed prime mover to produce variable frequency AC power which is rectified, filtered, and applied to the inverter as DC power over a DC link.
Ideally, the inverter controller should operate the inverter switches to minimize DC content in the inverter output. U.S. Pat. No. 4,882,120 discloses a system for controlling an inverter in such a way that DC content in its output is minimized.
The system in that patent uses a single inverter stage for inverting DC input into AC output. A single stage inverter requires the inverter switches to be able to manage large amounts of power. If a second inverter is added to a first inverter and is also controlled in such a way as to minimize harmonic content in its output, the inverter switches of the second inverter can share power with the inverter switches of the first inverter thus reducing the amount of power that any one switch must control which in turn can lead to a lowering of the power rating of the inverter switches. Furthermore, it is possible to control the second inverter so that additional harmonics are eliminated which results in less filtering resulting in turn in less filter weight. Under certain conditions, the savings in filter weight exceeds the weight added by the second inverter such that the overall weight of the system is reduced. Weight reduction is important when inverter systems are used in certain applications such as on aircraft.